 you could cause what's called a microclimate in the attic. So if you start trapping that hot, humid air in between the roof sheathing and this material, you can have a mold or a mildew problem. What happens is you create a little rainforest. Same thing can happen if you put it over the insulation and it has no way for the moisture to evacuate. Because that becomes the problem is you're trapping the moisture.